Build CPython 3.13 by default (#1950)

* Build CPython 3.13 by default

Enable building with CPython 3.13 by default, no longer requiring prerelease_pythons flag to be set true.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* README.md: Add note about free-threaded support

* build_selector_test.py: cp313t still behind flag

Even with 3.13 being enabled by default, cp313t is still behind a flag for now.

This commit updates the test accordingly.

* fix unit tests

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: mayeut <mayeut@users.noreply.github.com>

README.md

@@ -39,7 +39,7 @@ What does it do?
 
 <sup>¹ PyPy is only supported for manylinux wheels.</sup><br>
 <sup>² Windows arm64 support is experimental.</sup><br>
-<sup>³ CPython 3.13 is available using the [`CIBW_PRERELEASE_PYTHONS`](https://cibuildwheel.pypa.io/en/stable/options/#prerelease-pythons) option. Free-threaded mode requires opt-in.</sup><br>
+<sup>³ CPython 3.13 is built by default using Python RCs, starting with cibuildwheel 2.20. Free-threaded mode will still require opt-in using [`CIBW_FREE_THREADED_SUPPORT`](https://cibuildwheel.pypa.io/en/stable/options/#free-threaded-support).</sup><br>
 <sup>⁴ Experimental, not yet supported on PyPI, but can be used directly in web deployment. Use `--platform pyodide` to build.</sup><br>
 
 - Builds manylinux, musllinux, macOS 10.9+, and Windows wheels for CPython and PyPy